Compensated concentration testing system and method



June 28, 1938. s. L. CHRISTIE 2,122,364

COMPENSATED CONCENTRATION TESTING SYSTEM AND METHOD Filed June 5, 1936 2Sheets-Sheet l 33 1 ATTORNEY.

June 28, 1938. s. 1.. CHRISTIE COMPENSATED CONCENTRATION TESTING SYSTEMAND METHOD Filed June 5, 1936 2 Sheets-Sheet 2 IN VEN TOR S RE/v L.CHRIST/E BY ATTORNE).

Patented June 28, 1938 PATENT OFFICE COMPENSAT'ED CONCENTRATION TESTINGSYSTEM AND METHOD Soren L. Christie, Los Angeles, Calif. ApplicationJune 5, 1936, Serial No. 83,661

10 Claims.

My invention relates to a novel method and apparatus for measuring theelectric conductivityof fluids, usually liquids, to indicateconcentration of impurities or other substances therein,

or to indicate the electric conductivity of the fluid for any otherpurpose.

"For instance, the invention may be used to indicate the solid contentin a stream or body of water or may be used to indicate the presence offoreign matter in condensed steam, boiler waters, etc.

It is an object of the present invention to provide a nuvel system,usually of portable character, for measuring electric conductivity of Upto the present time it has been customary in conductivity testinginstruments to provide electrodes mounted in fixed relation to acontainer in which the liquid to be tested is posi- 0 tioned, themounting, and sometimes also the container, being part of theinstrument. However, I have chosen to illustrate the present inventionas a system utilizing an electrodecartridge oi the general characterdisclosed in my co-pending application Serial No. 29,575, Concentrationtesting apparatus and meth filed July'2, 1935. This cartridge is adaptedto be immersed in the fluid to be tested and is usually self-containedand independent of a fixed base. 30 In the apparatus to be hereinafterdescribed,

I have illustrated a visible indicating means in the form of acalibrated current-responsive meter which indicates the electricconductivity or related phenomena of the fluid by measuring the currentflow through current paths in the fluid between the electrodes of thecartridge.

In testing fluids of widely varying concentrations and temperatures, itis desirable to be able to calibrate the indicating means or meter inaccordance with the temperature of the fluid to 'be tested, and also tobe able to electrically change the conditions tending to send currentthrough the fluid between the electrodes.

45 It is therefore an object of the present invention to provide anapparatus for testing the concentration of a fluid which includes incombination with a current-responsive indicating means, a calibratingmeans for calibrating the 50 indicating means in accordance with thetemperature of the fluid being tested.

It is another object of the invention to pro: vide a system of thecharacter described in the above paragraph in which a calibrating meansin '55 the form of a calibrating impedance is connected in circuit withthe current-responsive indicating means.

It is another object of the invention to .provide an adjustment meansfor varying the potential applied to the impedance to calibrate the in-1 dicating means in accordance with the temperature of the fluid beingtested.

Another object of the invention resides in the provision of an apparatuswhich includes a pair of electrodes immersible in the fluid to betested, 10

a current-responsive indicating means, a callbrating impedance, anadjustment means for varying the potential applied to the impedance. aswitch means for selectively completing a circuit from the adjustmentmeans through the impedance and the indicating means, or completing thecircuit to the electrodes and the indicating means.

'It is another object of the invention to provide an apparatus of thecharacter described in the above paragraph which includes an adjustmentmeans comprising a voltage regulating means and a compensating means forvarying the potential applied to the impedance to calibrate theindicating means when the circuit is completed through the impedance,and for impressing potentials oi diflerent magnitude across theelectrodes when the circuit is completed to the electrodes.

It is anotherpbject oi the invention to provide an auxiliary voltageregulating means for varying-the potential applied to the electrodeswhen the circuit is completed thereto without disturbing the adjustmentof the main voltage regulating means after the indicating means has beencalibrated.

It is a further object of the invention to provide means for varying theresistance in the circuit to current flowing to the indicating meanswithout changing the potential applied to the circuit after theindicating means has been callbrated.

It is a still further, object of the invention to provide a novel methodof calibrating a directreading. current-responsive indicating means inaccordance with the temperature of a fluid to be tested for electricconductivity to determine the concentration of impurities or othersubstances in the fluid. 60

These and other objects and features of the invention will be made clearin the following of the specification.

Referring to the drawings,

Fig. 1 is a wiring diagram illustrating a comis bination of parts andthe relationship thereof comprising one form of my invention.

Fig. 6 is a face view of a dial for the indicating means embodied in theform of the invention disclosed in Fig.

My invention comprises in its broadest concept a plurality of electrodesII connected in a circuit it, which circuit includes acurrent-responsive indicating means I3 and a calibrating means M forcalibrating the indicating means l3.

With reference to Fig. 1, I show one form of my invention in which thecircuit [2 is supplied with current from a main potential source 15which, in this form of the invention, preferably comprises a storage ordry battery having rather high potential. A source of approximately 50volts has been found adequate in most instances.

In connection with the calibrating means l4, prefer to provide anadjustment means, generally designated by the numeral IE, for varyingthe potential applied to the calibrating means I4 and for impressingpotentials of different magnitudes across the electrodes l l. Theadjustment means i6 of this form of the invention preferably includes avoltage regulating means I! and a com pensating means I 8 shown in theform of a patch tiometer, the details of which will now be described inconnection with the various parts of the invention generally mentionedabove.

Connected to one terminal of the main potential source l5, such as thepositive terminal thereof, is a conductor I9 which in turn is connectedto a movable contact 20 of a reversing switch 2| including a pair ofcontacts 22, these contacts 22 being connected by conductors 23 and 24to opposite sides of an auxiliary potential source 25 which, in thisform of the invention,

preferably comprises a storage or dry battery. By this arrangement theauxiliary potential source 25 can be connected to the main potentialsource thereto depending on the position of the contact arm 2.iwith'respect to the contacts 22 of the reversing switch. The mainpotential source E5,

' the auxiliary potential source 25, and the reversing switch 2| thuscomprise the voltage regulating means I! of this form of theinvention.

Connected across the auxiliary potential source 25 as by conductors 2tand 21 is a potentiometer 28 having a movable iead 29 connected to areversing switch 30 comprising a contactor 3| and a pair of contacts 32and 33. The potentiometer 28 comprises the compensating means l3included in the adjustment means I6 of the invention.

One of the contacts of the reversing switch 30, such as the contact 3 2,is connected to the calibrating means l4 of the invention whichcomprises an impedance of suitable character, such as a resistance. Thecalibrating means or imped- IS in additive or subtractive relation beingconnected to the negative terminal of V .above and which may include apair of suitably insulated tubes 38 providing axial passages 39 in whichthe electrodes 1 I are positioned so as to be in contact with a fluid inwhich the cartridge 31 is immersed. The electrodes H are respectivelyconnected, as by insulated conductors 40, to the contact 33 of thereversing switch 30 and to the conductor 34 leading to the indicatingmeans l3, substantially as indicated. The conductors 40 in practice areof considerable length so that the cartridge 31 may be convenientlyimmersed in a container of fluid to be tested. It should be understoodthat although I have disclosed the electrode cartridge 31 as includingonly two electrodes I l, a greater number of electrodes may be used orvarious arrangements of the electrodes may be made to change the lengthof the current path through the fluid or to change the number of currentpaths available for sending current through the fluid, as disclosed indetail in the mentioned co-pending application.

In Fig. 2, I show a dial which is provided for direct reading of theindicating means l3, the dial being graduated to provide a plurality ofscales one of which, as indicated by the numeral 46, being thecalibrating scale of the indicating means. The other scales,respectively indicated by the numerals 41, 48, and 49, are graduated toindicate the concentration of foreign substances in a fluid, forinstance, in grains per gallon.

I have, shown a double pole switch 5| across the conductors 26 and 21between the auxiliary potential source 25 and the potentiometer 28, theswitch 5| being preferably of the push-button type, which completes thecircuit only while being manually depressed. This permits only periodicdrainage of current from the source during calibration or testing.

The operation of this form of the invention is as follows: In order tocalibrate the indicating means I3 to correctly measure the conductivityof fluids of widely varying concentration and of various temperatureswith the same indicating means, I preferto establish the temperature ofthe fluid as an arbitrary calibration medium, the scale 46 of the dial45 being graduated in degrees Fahrenheit, as shown in Fig. 2. With thereversing switch 2! closed, the contactor 3| of the switch 30 is movedinto engagement with the contact 32, either before or after thecartridge 31 is immersed in the fluid to be tested. This completes thecircuit between the main potential source l5 through the adjustmentmeans iii, the impedance It, and the indicating means l3. The adjustmentmeans I6 is then operated to vary the potential applied to the impedancel4 so that the needle 50 of the indicating means l3 p-oints to agraduation on the scale 46 which corresponds to the predeterminedtemperature of the fluid to be tested. This may be done usually bymoving the movable lead 29 of the potentiometer 28 which varies thepotential applied to the impedance l4, but if desired, the reversingswitch 2| may be reversed to aflord additional variation of thepotential applied to the impedance. The double pole switch is closedduring this operation. The calibration scale is so correlated relativeto the indicating means and the fluid to be tested that this singleadjustment will compensate both for voltage variations of the source andthe temperature of the fluid to be tested to calibrate the instrumentfor subsequent conductivity tests of fluid at the temperature to whichthe instrument was calibrated. The single adjustment featurecompensating for temperature as well as voltage variation is animportant part of this embodiment of the invention.

After the indicating means has been suitably calibrated, as abovedescribed, in accordance with the temperature of the fluid to be tested,the switch 30 is operated to engage the contact 33. This completes thecircuit from the main potential source I5 through the adjustment means16 to the electrodes II when the double pole switch 5| is closed, thusimpressing a potential across the electrodes. The current flowing in acurrent path between the electrodes, as shown by the arrows 52, isindicated by the indicating means l3 on one of the scales, such as thescale 41.

It'is not desirable, after the indicating means I3 has been calibratedand during the time that the fluid being tested, it is possible thatafter the indicating means has been calibrated in accordance with thetemperature of the fluid, the needle 50 may be off scale with respect tothe scale 41. I prefer, therefore, to provide in connection with thisform of the invention, a scale-selecting means 55 in the form of a'pairof shunts 56 and 51 bridging the indicating means I3 in parallelrelationship, as shown, there being switches 58 and 59 suitably arrangedto selectively bridge the indicating means l3 by either of the shunts 56or 51. The arrangement and magnitude of the shunts 56 and 51 and thescales 48 and 49 are such that when one of the scale-selecting switches,for instance, the switch 58, is closed, the position of the needle 50 ofthe indicating means will indicate concentrations on the scale 48, andwhen the other selector switch 59 is closed, the needle 50 will indicateconcentrations on the scale 49. As the resistances of the indicatingmeans It and the shunts 56 and 51 are extremely small compared with theresistance of the total circuit, especially compared with the resistanceof the fluid, the arrangement just described may be used withoutmaterially changing the characteristics of the circuit I2 previouslyestablished in calibrating the indicating means I3, so that the sametemperature setting may be used for all of the scales 41 to 49. Theshunts 56 and 51 in connection with the indicating means l3, providedesirable flexibility of the indicating means for testing fluids ofwidely varying conductivity without changing the characa compensatingmeans l8 including a potentiometer 62. As shown, the tapped primary ofthe transformer 6| provides a plurality of taps 63 which are engageableby a movable contact 84 which in turn is connected by a conductor 85 toone side of the line 60. The potentiometer 82 includes a movable lead 66which is connected by a conductor 61 to the other side of the line 60.Connected across the potentiometer 62 is an auxiliary winding 68 whichis preferably connected to the primary of the transformer 8| in additiveor subtractive relationship therewith through a reversing switch 69. Insome instances it is preferable that the auxiliary winding 68 comprise aportion of the primary of the transformer. In such an instance thepotentiometer 82 would be tapped directly across a portion of theprimary of the transformer 6|.

The voltage regulating means H, of this form of the invention,comprising the taps 63 and the movable contact 64, and the compensatingmeans l8, comprising the potentiometer 62 and its associatedauxiliarywinding 68, are operable to vary' the potential applied to the balanceof the c rcuit through the secondary of the transformer 6 I. Connectedto one end of the secondary of the transformer BI, is a movablecontactor III of a reversing switch II including a pair of contacts 12and 13, one of which, such as contact 12, is connected to one end of thecalibrating impedance I4, the other end thereof being connected to aconductor 14 which completes the circuit I through the indicating meansl3 to the other end of the secondary of the transformer- The electrodesII of the cartridge 31 are connected by conductors 15 and 16respectively to the contact 13 of the reversing switch II and to theconductor 14 leading to the indicating mechanism l3, substantially asshown. In this form of the invention I prefer to provide an auxiliaryvoltage regulating means generally designated by the numeral 18 whichincludes a switch 18 adapted to engage a contact 80 which is connectedin series with the contact 13 of the reversing switch, the movable armof the switch 19 being tapped into the secondary of the transformer GI,as shown, by a conductor 8|. In this form of the invention I provide adial- 45 which includes the calibrating scale 46, for calibrating theindicating means I3 in accordance with the temperature of a fluid to betested, and includes a plurality of suitablygraduated scales 82 and 83for indicating the concentration of a fluid to be tested in grains pergallon.

In the operation of this form of the invention, with the contactor ofthe switch H in engagement with the contact 12, the movable contactor 64is moved to selectively engage one of the taps 63 of the primary of thetransformer BI; and the compensating means I8 is adjusted, bymanipulation of the movable lead 66 of the potentiometer 62 and thereversing switch 69, to vary the potential applied to the impedance l4,causing the needle 5.0 of the calibrating means to assume a point on thescale 45 which corresponds to the temperature of the fluid to be tested.

After the indicating means I3 has been thus suitably calibrated, thecontactor '10 of the switch II is moved into engagement with the contact13 whereupon the circuit I2 is completed to the contacts, impressing apotential difference thereacross, sending current in a current paththrough the liquid so that the resistance of the fluid to the current isindicated by one of the scales, such as the scale 82 of the indicatingmeans I3.

.contact the taps 81.

If the concentration of impurities or foreign substance in the fluid isof such character that the needle 58 is found to be oif of the scale 82,the switch I9 may be moved into engagement with the contact 88 and theswitch II moved out of engagement with the contact I3, with the resultthat the potential impressed across the electrodes I I is changed to theextent that the needle 58 will be moved into reading relationship withthe scale 83. It will be seen that the auxiliary voltage regulatingmeans, comprising the switch I9 connected in tapped relationship withthe primary of the transformer BI, is operable to vary the voltageapplied to the electrodes without affecting the characteristics of thecircuit in general after the indicating means I3 has been calibrated.

Referring to Figs. 5 and 6, I show still another form of the inventionin which the circuit I2 is connected to a potential source which may bean alternating current supply line 85, the circuit including theadjustment means IS, the calibrating impedance I4, the indicating meansI3, the

electrodes I I, and the auxiliary voltage regulating means 18 disclosedin connection with the form of the inventionjust described. In thepresent form of the invention, the adjustment means includes a voltageregulating means II which comprises a tapped-primary transformer 86providing a plurality of taps 81 connected, substantially as shown, tovarious coils of the primary, there being a movable engager 88 adaptedto selectively The movable contactor 88 is connected by a conductor 89to the movable lead 98 of a potentiometer 9I, one side of which isconnected to the line 85 by a conductor 92, and the other side of whichis connected to one end of a resistance 93. The other end of theresistance 93 is connected to a conductor 94 leading from the left-handterminal of the primary of the transformer 86 to a switch 95 whichconnects this conductor 94 with the line 85, substantially as shown.

In this form of the invention the taps 81 are so spaced as to change theapplied voltage in steps just suflicient to compensate for varioustemperatures of the fluid to be tested. Each tap is marked with itscorresponding temperature. One

' of the taps, for instance the tap indicated by the letter C, isconnected to the central tap of the primary and comprises thecalibrating tap of this form of the invention, and is marked with acalibrating indicia. One terminal of the secondary of the transformer 86is connected to a reversing switch g, including a pair of contacts 91and 98. The contact!!! is connected to one end of the calibratingresistance I4, the other end thereof being connected to a conductor 99completing the circuit to the other terminal of the secondary of thetransformer 86 through the indicating means I3. In some instances, inthis form as well as in the previously described forms, the secondarywinding of the transformer may be tapped in a manner similar to thatshown and described to provide the voltage regulating means of theinvention. The auxiliary voltage regulating means 18 including theswitch arm I9 and the contact 88 is tapped into the secondary of thetransformer 86, in the same manner as disclosed in connection with theform shown in Fig. .3, by a conductor 8|. Likewise, the electrodes IIare connected respectively by conductors I88 to the contact 98 of theswitch 96 and to the conductor 99.

In Fig. 6, I show a dial 45 adapted for use in Suitable concentrationindicating scales I82 and I83 are likewise provided.

In the operation of this form of the invention the indicating means iscalibrated to the calibrating line I8I prior to regulating theadjustment means for varying the potential in accordance with thetemperature of the fluid to be tested as follows. ment with the contact91, the movable contact 88 is first moved into engagement with thecalibrating tap C of the primary of the transformer 86. The movable lead98 of the potentiometer 9| is then adjusted until the needle 58 of theindicating means registers with the calibrating line I8I on the dial 45.The switch 96 is then thrown into engagement with the contact 98, afterthe immersion of the electrode cartridge 81 in the fluid to be tested,and the movable contact 88 is then moved into engagement with that oneof the taps 81 which coincides with the temperature of the fluid to betested. The resistance of the fluid to the current flow between theelectrodes II is.

then indicated by the needle 58 on one of the scales, such as the scaleI82.

In the event that concentration of the liquid is such that the needle isoiT the scale I82, the switch 86 is opened and the switch arm I9 of theauxiliary voltage regulating means may be thrown into engagement withthe contact 88 which, as explained in connection with the form shown inFig. 3, changes the potential impressed across the electrodes II to theextent that the needle 58 is moved into indicating relationship with thescale I83.

It will be seen that in each of the three forms of the'inventiondisclosed and described, I provide a combination including a pluralityof electrodes immersible in the fluid to be tested; circuit meansconnected to the electrodes for impressing potentials of differentmagnitudes across the electrodes for sending current in current pathsthrough the fluid between the electrodes; indicating means in thecircuit responsive to current flowing in the current paths between theelectrodes; calibrating means in the circuit for calibrating theindicating means; adjustment means for varying the potential applied tothe calibrating means; and. switch means for .se-

lectively completing the circuit through the calibrating means orcompleting the circuit to the electrodes.

.It should also be clear that I have provided a novel method of testingthe conductivity ofa fluid utilizing a system including in a circuit acurrent-responsive indicating means, a plurality of electrodesimmersible in the fluid to be tested, and a calibrating impedance, themethod being:

(1) Applying a potential to the impedance;

(2) Varying the potential applied to the impedance to calibrate theindicating means in accordance with the temperature of the fluid to betested; and

(3) Sending current in a current path between the electrodes immersed inthe fluid, the resistance of the fluid to the flow of current thereinbeing measured by the indicating means to indicate the concentration offoreign substance in the fluid.

It should be understood, however, that the invention is not intended tobe limited to the details of construction disclosed in the three formsdescribed, for the reason that various other arrangements 01' the partsshown might be made and various modifications of the three formsdescribed might be devised, all coming within the scope of the inventionwhich should be interpreted only by the appended claims.

I claim as my invention:

1. In a device for testing the conductivity of a fluid, the combinationof: an electric circuit including a main potential source; a pair ofelectrodes immersible in the fluid to be tested;

, an indicating means in said circuit responsive to current flowingtherein; an impedance in said circuit; switch means for selectivelycompleting said circuit through said impedance or to said electrodes;and adjustment means for varying the potential applied to said impedanceto calibrate said indicating means when said circuit is completedthrough said impedance, and for impressing a proportional potentialacross said electrodes when said circuit is completed thereto, saidadjustment means including an auxiliary potential source, a reversingswitch connecting said auxiliary potential source in additive orsubtractive relation with said main potential source,

and a potentiometer connected across said auxiliary potential source andproviding a movable f cuit through said impedance or to said electrodes;

and adjustment means for varying the potential applied to said impedanceto calibrate said indicating means when said circuit is completedthrough said impedance, and for impressing a proportional potentialacross said electrodes when said circuit is completed thereto, saidadjustment means including a voltage regulating means and a compensatingmeans, said voltage regulating means comprising a transformer in saidcircuit and including a tapped winding, and said compensating meanscomprising an auxiliary winding connected to the winding of saidtransformer, and a potentiometer connected across said auxiliary windingand providing a movable lead connected in said circuit.

3. In a device for testing the conductivity of a fluid, the combinationof: a plurality of electrodes immersible in said fluid; a calibratingimpedance; an indicating means having a tempera ture scale thereoncalibrated to correspond to differences in conductivity of said fluid atdifferent temperatures; switch means for selectively connecting inseries circuit with said indicating means said calibrating impedance andsaid electrodes; and adjustment means for varying the potential appliedto said circuit by said potential source while said calibratingimpedance is in said circuit for varying the reading of said indicatingmeans until said indicating means indicates on said temperature scalethe temperature of the fluid to be tested, after which said switch meansis operable to disconnect said impedance from said circuit and insertsaid electrodes therein.

4. In a device for testing the. conductivity of a fluid, the combinationof an' electric circuit; a plurality of electrodes immersible in thefluid to be tested; an indicating means in said circuit responsive tocurrent flowing therein, said indicating means having a temperaturescale and one or more concentration scales thereon; an impedance in saidcircuit; switch means for selectively completing said circuit throughsaid impedance or to said electrodes; and adjustment means for varyingthe potential applied to said impedance to cause said indicating meansto indicate a point on said temperature scale corresponding to thetemperature of the fluid to be tested when said circuit is completedthrough said impedance, and for impressing a proportional potentialacross said electrodes to cause said indicating means to indicate theconcentration of the fluid being tested on one of said concentrationscales when said circuit is completed to said electrodes.

5. In a device for testing the conductivity of a fluid, the combinationof: an electric circuit; a plurality of electrodes immersible in thefluids; an indicating means in said circuit responsive to currentflowing therein said indicating means having a calibrating pointthereon; an impedance; switch means for selectively completing saidcircuit through said impedance or to said electrodes; voltage regulatingmeans including a temperature scale for changing the potential appliedto said circuit and including a calibrating indicia; compensating meansfor further varying said potential applied to said circuit while saidcircuit is completed through said impedance and while said voltageregulating means is set at said calibrating indicia to cause saidindicating means to assume a position corresponding to said calibratingpoint prior to adjusting said voltage regulating means to a temperatureon said temperature scale corresponding to the temperature of the fluidto be tested, thus calibrating said indicating means prior to completingsaid circuit to said electrodes.

6. In a device for testing the conductivity of a fluid, the combinationof an electric circuit including a potential source; a direct readingmultiscale meter responsive to current flowing in said circuit andincluding a temperature scale whereby said meter may be calibrated tothe temperature of the fluid to be tested; an impedance in said circuitfor calibrating said indicating means; adjustment means in said circuitincluding voltage reguiating means and a compensating means for varyingthe potential applied to said impedance; a plurality of electrodesimmersible in the fluid to be tested and connected in said circuit forreceiving an applied potential thereacross; and switch means forselectively completing said circuit through said impedance or completingsaid circuit through said electrodes.

7. In a device for testing the conductivity of a fluid, the combinationof: an electric circuit; a plurality of electrodes immersible in thefluid to be tested; an indicating means in said circuit responsive tocurrent flowing therein, said indicating means having a temperaturescale and one or more concentration scales thereon; an impedance in saidcircuit; switch means for selectively completing said circuit throughsaid impedance or to said electrodes; adjustment means for varying thepotential applied to said impedance to cause said indicating means toindicate a .point on said temperature scale corresponding to thetemperature of the fluid to be tested when said circuit is completedthrough said impedance, and for impressing a proportional potentialacross said electrodes to cause said indicating means to indicate theconcentration of the fluid being tested on one'of said concentrationscales when said circuit is completed to said electrodes; and scaleselecting means connectable in said circuit for causing said indicatingmeans to successively read on said concentration scales. 7 i

8 8. In a device for testing the iconductivity of a fluid, thecombination an electric circuit including a potential source and anauxiliary potential source; means for connecting said auxiliarypotential source to said potential source in 10 additive or subtractiverelation; a multi-scale in= dicating means responsive to current flowingin i said circuit; an impedance in said circuit for cali- Y brating saidindicating means; adjustment means W in said circuit including voltageregulating means and a compensating means for varying the potentialapplied to said impedance; a plurality of electrodes immersible inthe'fluid to be tested and connected in said circuit for receiving anapplied potential thereacross; switch means for selectively completingsaid circuit through said impedance 5 or completing said circuit throughsaid electrodes;

1 and scale selecting means cennectable in said circuit for causing saidindicating means to successlvely read on said scales thereof withoutvarying the potential applied to said circuit.

9. In a device for testing the conductivity of a fluid, the combinationof: an electric circuit;

voltage regulating means for regulating potential applied to saidcircuit including a transformer at} having a main winding and anauxiliary winding; means for connecting said auxiliary winding'to saidmain winding in additive or substractive relation; a multi-scaleindicating means responsive to current flowing in said circuit; animpedance in said circuit for calibrating said indicating means;adjustment means in said circuit including voltage regulating means anda compensating means for varying the potential applied to saidimpedance;a plurality of electrodes immersible in the fluid to be tested andconnected in said circuit for receiving an applied potentialthereacross; switch means for selectively completing said circuitthrough said impedance or completing said circuit through saidelectrodes; and 10 scale selecting means conneetable in said circuit forcausing said indicating rileans to successively readon said scalesthereof withoutyaryingthe primary potential applied to said circuit.

10. A method for testing the conductivity of a fluid by use if amulti-scale meter having a temperature scale thereon, a plurality ofelectrodes immersible in the fluid, a calibrating impedance,

and a voltage adjustment means, which method includes the steps of:connecting said meter'and g0 calibrating'impedance in series circuitacross a potential source; adjusting the potential sending currentthrough said meter until said temperature scale an said meter gives areading corresponding ts the temperature of the fluid to be tested;removing said calibrating impedance' ii'rom sai'jd circuit placing saidelectrodes in said series or said meter.

1 SOREN L. CHRISTIE.

circuit without changing the potential setting

